1. Field of the Invention
This invention relates to an improvement in an illumination system using optical fiber bundles, and more particularly to the expansion of the numerical aperture in an optical fiber bundle used for instance in an endoscope.
2. Description of the Prior Art
In an endoscope or the like in which optical fiber bundles are used for illuminating and viewing the interiors of intestines or the inside structures of instruments, illumination light is guided through an optical fiber bundle from an outside light source and is distributed to illuminate the object to be viewed at an end of the bundle. The area illuminated by the illumination light emitted from the end of the optical fiber bundle is determined by the numerical aperture (N.A.) thereof which is represented by the following formula EQU N.A.=(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2
where n.sub.1 is the refractive index of the core of the fiber and n.sub.2 is the refractive index of the fiber cladding. The numerical aperture (N.A.) is usually about 0.5 and the angle of the extreme meridional ray trapped in a fiber is about 60.degree.. This means that the illumination light emitted from the end of the optical fiber bundle has a distribution angle of about 30.degree. which is the half angle of the cone of light at the exit of the optical fiber bundle.
An expanded distribution angle of illumination light is sometimes desired so as to make it possible to obtain a broader area of illumination on the object and enable observation and photographing of the object with a wider field of view.
In order to expand the distribution angle of illumination light of the optical fiber bundle, it has been suggested to make the exit end face of the bundle convex or concave as disclosed in Japanese Utility Model Publication No. 10346/1977 issued Mar. 5, 1977. However, an optical fiber bundle having the convex or concave exit end face is very difficult to produce.